[PeterisP]

This works because the interests of developers of the dependency and the dependent part are aligned, or can be aligned/decided in the case of conflicting interests.

For the rest of us, we need the practical ability to simply make changes that might break dependencies, because of that the ability to fixate to specific old versions, and the ability to create alternate/forked incompatible versions - simply because it allows people to go/develop in the direction they want without being tied down by others who may want a different direction or even no direction at all, because it's not maintained anymore.

The main reasons (as usually is the case) are social/political, not technical.